1^26.] Barometrical Measurement of Heights. 95 



air. Ramond (or it may be an error of his translator in reducing 

 metres to feet) states the correction per degree C, at the mean 

 pressure of the atmosphere to be more than 3-lOOths of an inch, 

 and equal in elevation to more than 3 feet. The correct quanti- 

 ties are 5-lOOOths of an inch, and more than 4 feet. The dila* 



tation of TTTT. used by Ramond has been discovered to be incor- 

 5412 •' 



rect from a mistake in the calculations of the experimenters. 



Granting the assigned values of the dilatation of mercury and 



brass to be correct, if we suppose one of two syphon barometers, 



alike in every respect in their construction to be placed at any 



elevation on the side of a mountain, and the other exactly 423 



feet above it, the mercury of the instrument at the base being 



constantly maintained at the freezing point, and that of the supe- ^ 



rior one at 212° F. then would the observed heights of the mer^^^ 



curial columns (the air being at 32°) be aiikey without regard td** 



the atmospheric pressures or variation of the pressures they'^ 



supported. '^.W 



Correction for the Aqueous Vapour contained in the Atmospheres^ 



An atmosphere of aqueous vapour uniformly of the same 

 temperature would decrease in density in geometrical progression! 

 for equal perpendicular ascents; but as its specific gravity conjrf. 

 pared to dry air is as 10 to 16, it follows that if its density at 

 an altitude of 16,000 feet should be found to be diminished one- 

 half, a decrease in the same ratio of the density of the dry air 

 would take place at an elevation of 10,000 feet; the pres- 

 sures of the two fluids at the base being the same, and theiiTj 

 temperatures alike. 



An equal weight of dry air being mixed with the vapour, the 

 two fluids would exist as distinct atmospheres, the particles of 

 the one not pressing on those of the other,* and consequently 

 maintaining their peculiar arrangement of density undisturbed. 

 At the base the pressure would be double its former value, but 

 as the altitude increased, this ratio, for the reasons assigned, 

 would continue to diminish. Should the temperature of the 

 mixture decline in proportion to the altitude, the diminution of 

 the density of the vapour would be more conformable to that of 

 the air ; — the decrement of temperature being sufficiently rapid, 

 it might even exceed it. So numerous and variable are the 

 causes tending to disturb any regular law of the variation of the 

 density, that we may consider the mean density of a stratum of 

 moist air as equal to that of one of dry air under the same pres- 

 sure, but of a temperature superior to that indicated by the 

 detached thermometers, by the mean of the equations for the 

 observed dew-points at the two stations, computed as pointed 



* Dalton. " '' - - > o 



